I've never been certain of the rules surrounding the use of the -i suffix for pluralizing a word. I had thought that it was used for any word whose singular ended in an 's', but that doesn't appear to always be true.

For example, the plural of octopus can be written octopi. But the plural of chorus seems to always be choruses, never chori.

The short answer is "only if you've seen the plural formed using -i by a respected source". In all other cases, make your plural by adding -s. If you happen to be picked up on this now and then, you can reasonably suggest you're just "ahead of your time", since it's highly likely all plurals will eventually be regularised.
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FumbleFingersFeb 21 '12 at 16:28

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@FumbleFingers Agreed, but I’d modify what you said to say either-sor-es depending on the regular rules of English plurals.
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tchristFeb 21 '12 at 17:50

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The plural of 'octopus' is 'octopuses' or 'octopodes,' not 'octopi.'
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Robert S.Feb 21 '12 at 17:56

@FumbleFingers I have a whole theory about that. We use -s/-z/-ɨz for either of the plural or possessive inflection, but not both. If it would be doubled, we surprise it. Also -iːz words are left alone. Hence 2 series’ ends,James’s pal,Mr Jones’ shirt,the Joneses live at the the Joneses’ farm,these series’ starts,Diogenes’ lamp etc. It’s ’cause folks are hung up writing instead of speaking that they are always getting these things spelled wrong, which is so strange because they by definition never say them wrong. But they think spelling trumps speaking, letting the tail wag the dog.
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tchristFeb 21 '12 at 20:58

4 Answers
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Use "-i" for plurals when the word is borrowed from a Latin word that used "-i" for plurals. Note that in Latin not all words that end in "-us" are made plural by changing the "us" to "i". For example, in Latin the plural of "locus" (place) is "loci", but the plural of "fructus" (fruit) is "fructus".

Octopus comes from Greek and not Latin, and so does not follow the Latin rules.

@tchrist Are you saying that a word could be made plural by adding "-i" even if it doesn't come from Latin? Sure, I suppose my answer is poorly worded in that sense. Latin isn't the only possible source of "-i" plurals, just one source. Surely the most common source in English, but not necessarily unique.
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JayFeb 21 '12 at 18:38

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Yes, that’s what I was saying. I voted for you anyway. :)
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tchristFeb 21 '12 at 18:41

Similarly, there are plenty of -us nouns that do not go to -i plurals. Many of these are from the Latin 3rd declension, and some are from its 4th. Others aren’t from Latin nominative singulars at all, like onmibus, a dative plural, and ignoramus, which was a verb in the 1st person plural present indicative active (the Romans were really tense, you know).

Among other reasons it depends on the root being Greek or Latin, however there are many reasons for the ending being us/i as seen in this wikipedia entry which in turn is heavily discussed in the rest of this post, so I recommend you take the Wikipedia entry cum grano salis/άλας ;)

The term "octopus" is from Greek ὀκτάπους[37][38] (oktapous, "eight-footed"), with traditional plural forms "octopuses" (pronounced /ˈɒktəpʊsɪz/) from English grammar and "octopodes" (pronounced /ɒkˈtɒpədiːz/) from the Greek. Currently, "octopuses" is the most common form in both the US and the UK.

Some authorities find that octopi is an objectionable hypercorrection, feeling that the form arose from the incorrect assumption that "octopus" is a Latin 2nd declension form.

However, "octopus" is a Scientific Latin 3rd declension noun with a plural of octopodes. Nevertheless, the Oxford English Dictionary lists "octopuses", "octopi", and "octopodes" (in that order), labelling "octopodes" 'rare' and noting that "octopi" derives from the misapprehension that octōpus is a second declension Latin noun.

Is there a way to determine a word's root as being Greek or Latin?
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Ian C.Feb 21 '12 at 16:03

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Etymology dictionary... But most dictionaries will have the plural after the word
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mplungjanFeb 21 '12 at 16:06

Several. One can learn Greek or Latin, for instance. Or one can look up the word in a dictionary or a book of Greek and Latin roots (there are a number). Further information available here.
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John LawlerFeb 21 '12 at 16:07

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@IanC. Actually, it’s a great deal more complicated than this. You can’t just claim Latin vs Greek. That’s so oversimplified as to be wrong.
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tchristFeb 21 '12 at 17:47

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I’ll bet you that no Roman ever uttered the word octopus, let alone made it a plural. It was coined by Linnaeus during the 18th century. I’d like to see the source the pretends that it was from the 3rd declension, whose -us words ended in -ora or -era in the plural. There were Greek irregulars thrown around the Latin declensions, but this is one the Romans never used.
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tchristFeb 21 '12 at 18:15

O tempora o mores! This is false. Please do not propagate the myth. Only second declension masculine nouns ending in -us ever went to -i. Many others did not, like those in the 3rd declension: corpus/corpora, opus/opera, genus/genera, tempus/tempora. There are also 4th declension nouns where -us > -ūs, like apparatus/apparatūs, status/statūs, hiatus/hiatūs. Plus there are things like cornū/cornūs whose nominative plural was cornua. And what declension do you think magna Venus was in? If you had more than one of her, they would be magne Venera; cf. venereal.
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tchristFeb 21 '12 at 17:19